Topic: Resurrected HP-55

I bought this unit SN 1604A 02414 on Ebay, rated 'as is' for parts.
It was DOA.

Internal inspection : keyboard dead with no chance to resurrect : little green powder everywhere.
Soak it in Vinegar and Water : no cure,
Front side : clean except silver trim,
Back side : battery door stops missing and black labels previously and badly peeled at both upper corners,
- NO LED display,
- keys OK  snap properly. 
- one battery contact missing the other cut off,
- leakage in the battery compartment
- HP carrying case Ok, zipper works,
- No manual but, the green Quick reference guide was there.

I routinely check the PSU : all voltages were there.
Master clock from Anode Driver on the scope and signals IS, IA look good.
Two phase Mos clock was there.

I assumed C&T and Clock Driver were ok.

I could not swap display with a HP35 display PCB (not compatible).
I check the LEDs directly : some segments ok, others dead.

I routinely assumed that Anode & Cathode drivers were ok : there are rather robust and I saw the master clock, an output of the AD.
I swapped the A&R chip with one from an HP35 : no avail.
So I put the blame on the keyboard, the Leds and the bad luck and waited the opportunity to buy another wrecked 55.

It happened that a friend -Dan W.- had a known almost good display PCB with a only a bad Led cluster.
We swap parts over the ocean.

I took some vacation and forget about the 55.
Returning home, I found the pack and I plugged the 55 logic board into the Display PCB.
It worked, except for the bad segment of the bad cluster.

Then I extracted (the Weller pump worked hard this day) a LED Cluster from a HP-35.

The HP-55 came to life.

I had a little alignment problem quickly solved but it remains a small issue with the LEDs that appeared to be dimmer on the 35 (at least the one I had).

Then I decided to do some experiments.

I changed the anode driver and put a genuine HP-35 chip (1820-1029)

Return on experience : on this HP55 the A&R and the Anode Driver are organs from a HP-35 !

I reconstructed the battery contacts, always from the same salvaged HP35.
I fixed the spot welded the tiny metal strip with the tip of my iron (it's enough).
But probably my solders (on the black and red wires) were not flat enough.

I had one incident : I forgot the 'scotch tape' strips they put at the back of the black plate.
That blunder brought a ground on Vcc and killed one diode in the PSU.
Note: in this situation the Cathode Driver that works between Vbatt and Vcc was heating a lot.
Ok, I fixed Vcc (took diode 1N270).

I re soldered  the new LED cluster too fast and reproduced the so called 'ghost' effect (cross cluster this time) ; cleaning my soldering solves this new problem.

Finally :

- I reconstructed the battery door, using two lockers from the same 35,
- I repainted the silver trim :

For the top bar I used a method learnt from Dan W :
mask almost all the calc with a plastic bag up to a point above the switches, I finished the protection mask with scotch tape.
I did the same for the zone above the bar using plastic and scotch tape.
Must be very hermetic.
One blow of silver painting is enough.

For the side and lower trim, I used a thin brush.

The recovery is not perfect but very good.

It just need new 'back labels'.

And one last thing : When you re glue the black label don't put in place the 2 screws underneath the label.
The 4 others remaining will do the job ok. 

Re opening the calculator -in the future- won't hurt the black back label.

Here is the front view of the calculator before the cosmetic restoration.

And the back view.

back view after restoration